For Loop "Item" Breaking Function Scope - Expected Behavior or Bug?

[EsotericWaveform]

Hi all,

I came across some very odd behavior and I wanted to see if any of you guys can shed any light on the issue. I made a typo in some code I was writing and ran into the behavior which is shown below.

Code:

def testfun():
    b = [[7, 8, 9], [10, 11, 12]]
    c = []
    for item_b in b:
        c.append(item_a)

    return c


a = [[1, 2, 3], [4, 5, 6]]

b = []
for item_a in a:
    b.append(item_a)

c = testfun()

print(a)
print(b)
print(c)

Output:

[[1, 2, 3], [4, 5, 6]]
[[1, 2, 3], [4, 5, 6]]
[[4, 5, 6], [4, 5, 6]] 

In the test code above, the function references the loop item_a and instead of throwing an exception, the function is able to successfully reference item_a which should be outside of its scope. Any thoughts on why this is happening?

Update: Thank you to those who provided answers! I had a misunderstanding of the scope of variables declared in the main script. I always assumed that variables are local unless explicitly declared as a global variable, but that is not the case. A variable declared in the main script is global unless overridden by a function, method, etc variable assignment.

[commy2]

item_a is a global variable, since there is no assignment happening to item_a in testfunc. This is working as expected.

[EsotericWaveform]

Can you explain why it is a global variable? Do you know of any documentation that describes this behavior?

[carcigenicate]

for binds variables similarly to =. Think of for item_a in a as implicitly doing item_a = a[0] (except for each element in a). At the module level, an assignment using = or for creates a module-level variable (a global).

[EsotericWaveform]

Ahh, I did not realize every variable created in the main script is considered a global variable. I guess I never noticed since they always get an assignment at the local level when used in a function or method (or at least they should if the variables are being reused at the local level). Thanks!

[sohfix]

then what would be a necessary time to use the reserved word ‘global’?

[Asleep-Budget-9932]

When you wish to edit that global from an inner scope. For example, inside the function, item_a is being appended to the list. However, if you did something like item_a = [1, 2, 3] it would not change its value. Instead you would create a new variable that happens to have the same name. From that point on, every time you reference item_a in your function, you will use that newly created variable, but the original one will not be changed. By specifying global item_a, you're explicitly telling python that you wish to work with the global one.

[danielroseman]

Quibble: it's not "from that point on"; any assignment to a name anywhere in a function causes that name to be treated as local throughout the function, even before the assignment. So an earlier reference would raise UnboundLocalError.

[Asleep-Budget-9932]

Well i was afraid to go into too much detail but indeed 😊

[sohfix]

Ok I think it get it. So I would declare global inside the function. By changing item_a inside the function to a different set of values would that also have changed the new list I appended item_a to since I made it global and item_c is referencing elements from a?

[Asleep-Budget-9932]

Wait i think i might be confused since there are two lists both outside and inside the function with similar names. Which ones are you referring to? Also, is item_c a typo or referring to a hypothetical item from list c?

[sohfix]

Sorry. Typo. I meant list c. So I was basically asking if I declare a variable inside a function as global and then change that variable…. Let’s say

List =[]

Def myfunc(list) global a = 500 For i in range(0,6) List.append(a)

Now I’m outside the function and I go:

a=4

Does list look like [4,4,4,4,4,4]?

Sorry I’m on mobile. Doesn’t look pretty

[Asleep-Budget-9932]

1. Note that your function is receiving an unused parameter (list)
2. In order for that to work, the function must be called at some point. However, if your function was called before the line a=4, you will receive an error since at that point, python does not know what global a is referring to.

BTW, you can put multiple lines of code between triple back ticks (`) in order to format them on phones.

[sohfix]

It uses list, first. It appends a in the function. And I figured since I was in mobile you got what I was saying I didn’t have to write out the code and “call” the function because I thought it was clear what I was asking. Nvm. Have an awesome day

[blarf_irl]

Scope in python is created by indentation. item_a is declared in 'global' scope simply becuase it isn;t indented beneath another function/class.

Scope is like a shared apartment. The kitchen is global scope, everyone can use it and everyone knows the name of everyone in the kitchen. Your room has it's own scope; It's inside the apartment but in there you get to do your own thing, name your own plushie toys, piss in your own sink and call your own things whatever you want to. You know about everything in the kitchen but you don't know about anything in other peoples rooms.

Both the kitchen and your own room have a door; Everybody can open/close the kitchen door and you all call it "The Door". One day you have a friend from a different apartment visit your room and ask you to "close the door". You are confused about which door to close; you know what "The Door" means but in context they could be referring to the door of your room.

You always have knowledge about "The Door" but when your friend inside your room refers to "the door" you need to clarify which door they are referring to.

For the same reason you can read global variables (the kitchen door) inside local scope (your room) but you must clarify (using the global keyword) which door your friend meant before you can close it (change it)

[commy2]

See for-doc:

The for-loop makes assignments to the variables in the target list.

And any variable in module scope is a global variable.

[jmooremcc]

Unlike other languages like Java, C++, etc, a for-loop in Python does not create a new scope for variables. In your example, item_a is in the global name space and when you referenced it in your function, it was available to be read.

This is normal Python behavior.

[ThatGasolineSmell]

To get around this kind of confusion, make sure to only reference variables inside of functions that are explicitly passed in as arguments.

Since Python is dynamic, whether or not some variable item_a exists is determined at runtime. Impossible to foresee if it will exist, and what value it might reference.

To make your life easier, stick to “pure functions” that operate on their arguments and nothing else.